Phorbol esters as probes of the regulation of thyrotropin secretion

12-0-Tetradecanoylphorbol 13-acetate and thyrotropin releasing hormone were compared as thyrotropin secretagogues in cultured rat pituitary cells. The maximal secretion evoked by the former was only half that evoked by the latter. A23187 plus 12-0-tetradecanoylphorbol 13-acetate together produced as large a response as thyrotropin releasing hormone. Triiodothyronine inhibited phorbol ester-induced thyrotropin secretion, and this inhibition required protein synthesis.